1. Technical Field of the Invention
The invention relates generally to a bi-directional data communication link in a cable television system, and particularly to an increased downstream data transmission capacity in a system having a control channel of the so-called out-of-band type for transmitting information from the central equipment to the terminals.
2. Discussion of Related Art
A cable television system is usually a tree-like distribution network having at the root a main amplifier or the so-called head-end of the operator which controls the network. More generally the main amplifier can be called the central equipment. The distribution lines are branched in a tree-like fashion towards the terminals of the subscribers, of which there can be even hundreds of thousands under the same central equipment. In order to prevent signal weakening and to reduce interference the distribution lines have distribution amplifiers, repeaters and other devices known per se.
Plans to change the cable television systems from unidirectional distribution networks into bi-directional data transmission networks have been presented recently. Then the data transmission direction from the central equipment to the terminals is generally called downstream, and the opposite direction is called upstream. In addition to the high capacity downstream channel the system comprises at least one upstream channel, through which the terminals can transmit information towards the central equipment, and a downstream control channel with a relatively low capacity, through which the central equipment controls the use of the upstream links. The control channel can comprise cyclically repeating fields which the central equipment multiplexes into the digital video picture or other signal transmitted on the main channel by utilizing its frame structure. Such a control channel is of the so-called in-band control channel type. In another embodiment the control channel is located on a frequency band of its own, whereby it is a so-called out-of-band control channel. This invention relates primarily to systems where the control channel is of the out-of-band type.
The invention can be applied e.g. in digital video systems known per se, i.e. DAVIC (Digital Audio Visual Council) and DVB (Digital Video Broadcasting). Definitions relevant to the invention are presented in the publications "DAVIC 1.0 specification part 08; Lower layer protocols and physical interfaces, December 1995", "DAVIC 1.0 corrigenda part 08; Lower layer protocols and physical interfaces, Edited version after New York meeting, Rev. 2.1, June 1996", "DAVIC 1.1 specification baseline document #18. Cable modem baseline document, Rev. 2.0, as of New York meeting, June 1996" and "ETSI draft specification prETS 300 800; Digital video Broadcasting (DVB); DVB interaction channel for Cable TV distribution system (CATV). TM 1640 Rev. 4, June 1996". The cable television system presented in the publications can be based on coaxial cables or at least partly on optical fibers, whereby in the last mentioned case it is also called an HFC network (Hybrid Fiber Coax).
FIG. 1 shows the proposal in the publication prETS 300 800 for dividing the frequencies in the DVB system. The dimensions on the horizontal axis in the figure are illustrative, and the vertical axis only shows which signals are directed from the central equipment to the terminals (DS, upwards in the figure) and which signals are directed in the opposite direction (US, downwards in the figure). The frequency band 101 extends from about 50 MHz to almost 900 MHz, and typically it is divided into channels 102 with a typical width of 6 to 8 MHz, of which only three are shown for the sake of clarity. Each channel carries one QAM modulated (Quadrature Amplitude Modulation) signal, which can contain one or more digital video signals in the MPEG-TS format (Motion Picture Experts Group--Transport Stream) or other information requiring a high transmission capacity. The frequency band 103 extends from 70 to 130 MHz, and it contains channels 104 with a width of one or two MHz, each carrying one QPSK modulated (Quadrature Phase Shift Keying) control channel. Also here only three channels are shown for the sake of clarity. The frequency band 105 extends from 300 to 862 MHz, and its contents corresponds to that of the frequency band 103. In the figure the frequency band 106 extending from 5 to 65 MHz is reserved for the upstream links, and it contains channels 107 of which the figure shows only three, each having a width of 200 kHz, 1 MHz or 2 MHz. It is planned to use the QPSK modulation also on these upstream links. The object is to use QPSK modulation also on the upstream links. However, the new definitions for the DAVIC system also provides for the use of QAM modulation on the low capacity channels.
The communication on each upstream channel is divided into nine cyclically repeated time slots. The central equipment determines how these time slots are used, so that some of the time slots can be used for ranging, which aims at the measurement and compensation of transfer delays, and some of the time slots can be freely used by the terminals in a kind of reservation contest, and according to a particular reservation list some of them are assigned to be used by terminals which have made reservations, and some of the time slots are used according to a time schedule, which guarantees a certain regular data transmission capacity for a link. In the DAVIC system the central equipment transmits on one downstream control channel information regarding the use of up to eight upstream channels. The transmission of this information on the downstream control channel is described in more detail below.
According to FIG. 2 the transmission on the control channel comprises SL-ESF frames 108 (Signalling Link Extended Superframe). The length of one SL-ESF is 4632 bits and it is divided into 24 frames of 193 bits. In FIG. 2 the frames are numbered sequentially from 1 to 24, and further one frame is enlarged and marked by the reference numeral 109. At the beginning of each frame there is a so-called overhead bit 110, which is followed by the payload section 111 comprising 192 bits. The significance of the overhead bit depends on which frame in the SL-ESF is considered. In the SL-ESF frames with the sequential number 4, 8, 16, 20 and 24 the overhead bit has a fixed value and functions as a frame alignment bit. Correspondingly, when the overhead bits in the frames 2, 6, 10, 14, 18 and 22 are placed in row they form a CRC checksum, which represents the bit contents of the previous SL-ESF. In every second frame the overhead bit starting from the frame number 1 is a so-called M counter, which indicates the timing of the time slots in the upstream channel controlled by this control channel.
The payload of the SL-ESF is obtained by writing the payload sections of all frames in sequence, beginning with the payload section of frame number 1 and ignoring the overhead bits. The SL-ESF payload is usually written in the form of the table below:
Row No. 2 bytes 53 bytes 2 bytes 1 byte 1 byte 1 R1a R1b ATM cell RS 2 R1c R2a ATM cell RS R2b 3 R2c R3a ATM cell RS 4 R3b R3c ATM cell RS R4a 5 R4b R4c ATM cell RS 6 R5a R5b ATM cell RS R5c 7 R6a R6b ATM cell RS 8 R6c R7a ATM cell RS R7b 9 R7c R8a ATM cell RS 10 R8b R8c ATM cell RS T T
In the table the rows 1, 3, 5, 7 and 9 each have 57 bytes, the rows 2, 4, 6 and 8 each have 58 bytes, and the row 10 has 59 bytes. When the bytes Rxa-Rxc (where x is one of the integers 1 to 8) are placed one after another they form a field with the length of 24 bits, which contain information regarding the upstream channel defined by the integer x. If the downstream channel controls only one upstream channel, then the bytes, except the bytes R1a, R1b, R1c, are insignificant. Of said 24 bits the first bit instructs, when required, the terminals to measure and compensate for the time delays, the next six bits indicate the use of the time slots in the upstream channel, the next nine bits transmit acknowledgements of frames received earlier by the central equipment, the next two bits enable or disable capacity reservation attempts made by the terminals, and the last six bits form a checksum regarding the previous bits.
Each row contains an ATM cell of 53 bytes, which can contain information directed to a certain user, such as instructions (so-called MAC messages; Medium Access Control) regarding compensation of timing errors and distribution of upstream capacity, or transmitted information relating to an application. In order to detect and correct transmission errors the frame structure contains a Reed-Solomon code with a length of two bytes, which is marked RS in the table. The two bytes marked with T form the trailer of the payload.
The aim of the above presented arrangement, in which one downstream channel controls up to eight upstream channels, is that it is used mainly so that the control channel transmits very little information in addition to information relating to the use of the upstream channels. Actual downstream data transmission is carried on the high capacity QAM modulated main channels, through which a user can load a file to his or her terminal, for instance.
There will be a problem if a user wants to use his or her terminal for several purposes simultaneously. As an example we can examine a situation, in which the terminal in family use is a digital multimedia terminal, a so-called Set Top Box, which is connected to a large-sized display and has further an interface for a home computer. An arrangement of this type is shown in FIG. 3, where the reference numeral 112 represents the central equipment of a cable television operator, the reference numeral 113 represents a digital multimedia terminal, and the reference numeral 114 represents a home computer. The central equipment contains a plurality of main channel transmitter units 115, a plurality of control channel transmitter units 116, and a plurality of receiver units 117 for the upstream channels. The user terminal contains a tuneable receiver 118 for reception of the high capacity main channels, a second tuneable receiver 119 for reception of low capacity control channels, and a transmitter unit 120 in order to generate the upstream transmissions.
In the situation of the example the largest part of the family members want to watch a movie transmitted in a digital form (e.g. in the MPEG form; Motion Picture Experts Group) via the receiver, but one family member wants to utilize the bi-directional nature of the data transmission network and to scan through WWW pages (World Wide Web). In principle there are two alternative prior art solutions. According to the first alternative the operator equipment multiplexes the data required by the WWW user in the same downstream channel with the video picture. In the second alternative the operator reserves a whole high capacity QAM modulated channel for the video picture transmission and packs the WWW data into the ATM cells, which are transmitted on the downstream control channel.
The disadvantage of the above first mentioned alternative is that it requires facilities which present-day operators generally do not possess. In many cases the operators do not at all want to deal with the contents of the data transmitted on the high capacity downstream channel. The program is received as such from a satellite or other corresponding link, and the operator only controls its distribution via the cable network or the HFC network. In the second alternative the transmission of the file format data on the downstream control channel is easier to arrange according to present-day definitions and practices, but then the whole downstream capacity may be consumed. The situation is worsened by the fact that the relative number of low capacity channels is defined in the wrong way considering a typical home computer network user. For instance, data transmission between the home computer and the data network in order to scan WWW pages or to load public domain software is clearly asymmetric: the user transfers from the network to his or her computer amounts of data which are much greater than in the opposite direction. To meet this need there should be more downstream channels than upstream channels, rather than the opposite.
In another alternative embodiment the downstream data transmission is not even processed in the same device of the Set Top Box type, which is used to receive the main channel, but for this purpose there is a separate cable modem. Then the downstream data transmission can be totally separated from the transmission of the high capacity video picture. However, to a user it is frustrating to purchase separate devices, if the same object can be attained by one sufficiently versatile device.